Silica pigments and preparation thereof



United States Patent 3,307,906 SILICA PIGMENTS AND PREPARATION THEREOFOliver W. Burke, Jr., Fort Lauderdale, Fla. (P.0. Box 1266, PompanoBeach, Fla. 33062) No Drawing. Filed May 24, 1965, Ser. No. 458,480 12Claims. (Cl. 23-182) This invention relates to silica pigment materialsand to the preparation thereof from alkali metal silicate, and aimsgenerally to improve the same.

OBJECTS Particular objects of the present invention, severally andinterdependently, are to provide an improved process for the preparationof silica pigments; to provide a process for controlling the relativesizes of the primary particles of colloidal silica and of the aggregatesand flocs thereof which form the silica pigment particles, e.g. for theproduction of improved silica pigment materials; and to provide animproved silica pigment having useful characteristics and at aneconomical cost.

Other objects and advantages of the invention will be apparent from aconsideration of the herein set forth general and specific descriptionof illustrative embodiments thereof.

of silica pigment by the acidulation of alkali metal silicate eg sodiumsilicate, especially with carbon dioxide as acidulating agent, and ischaracterized by the addition, at certain predetermined stages duringthe progress of the acidulation, of water essentially free ofelectrolyte.

The course of the acidulation of sodium silicate solution to precipitatesilica therefrom can be classified in several distinct stages. Prior toacidulation, of course, is the preparatory stage in which the sodiumsilicate is diluted with water and in which the factor x designating theratio of SiO; to Na O in the sodium silicate formula Na O(SiO remainsunchanged. As a practical matter, in commercial sodium silicate used forthe production of precipitated silica, the value of x lies in the rangeof about 3.0 to 3.4, e.g. 3.22 in the 41 B. sodium silicate of commerce.I

The first period in the acidulation is termed the preprecipitationperiod, during which the silica of solution is polymerizing to formcolloidal particles, that is, the primary silica particles alsosometimes called the ultimate silica particles. In this period, noappreciable number of colloidal silica particles have aggregated to formsilica network particles. The end of this period is taken to extend to apoint at which the x value lies in the range of 4.2 to 5.3 (depending onthe conditions present). The present invention aims to provide a methodof controlling the size of network particles formed in the second stagein relation to the size of the colloidal particles produced in the firststage.

The second stage is termed the incipient precipitation period. Thisperiod embraces a region in which the colloidal silica particlesaggregate to form network particles, and is detectable by the appearanceof Tyndall effect, which usually occurs between x values of 4.2 and 5.3.

The term Tyndall effect is used herein in the same manner as defined inJ. Thewliss Encyclopaedic Dictionary of Physics, Pergamon Press, 1962,in which it is defined as follows:

Tyndall efiect.The scattering of light by very small particles, thescattered light being predominantly blue.

This authority further states that: This phenomenon and its dependenceon the size of the scattering particles was first investigated byTyndall. The scattered intensity is proportional to the square of thevolume of the particle and proportional to 10$. Thus the scattering forlight at the blue end of the spectrum is about ten times as great as forlight at the red end.

Under appropriate conditions the appearance of the Tyndall effect maymark the commencement of the second stage. This second or incipientprecipitation period continues to the point where precipitation ofsilica commences and preferably less than 10% of the silica has appearedas precipitate. This incipient precipitation period is deemed to extendfrom the point at which x has a value of between 4.2 and 5.3 to an xvalue of about 6.4 or higher (depending on the conditions present). Thepresent invention aims to provide a method of controlling the size ofthe network particles formed in the second stage independent of the sizeof the primary particles formed in the first stage.

The third period or the principal precipitation period of theacidulation is that period in which the major part of the precipitationof silica occurs and extend from the upper end of the second period whenthe precipitation of silica is commencing to the point at which theprecipitation of silica is substantially complete, which is normallyconsidered to be the point at which about 70% of the stoichiometricquantity of acidulating agent has been added and at which x has a valueof about 10 or more (which value may also vary with the conditionspresent). Depending on the conditions present, the extent offlocculation of the precipitate may vary.

The present invention has disclosed that particular advantage isattained by a process for the precipitation of reinforcing silicapigment by the gradual acidulation of an aqueous solution of alkalimetal silicate, especially sodium silicate, with the aid of carbondioxide, accompanied by a timed addition of water essentially free ofelectrolyte in an amount in the range of 10 to 150 parts or more perparts of water present just prior to such acidulation, such timedaddition being effected after an x-value of 3.75 has been reached, andpreferably after the appearance of a Tyndall eifect in the solution. Bythis new process the network particle size and/or the size of floc ofthe silica pigment are established relative to the size of the primarysilica particles thereof in a controlled or predetermined manner, thuscontrolling the characteristics of the pigment produced.

In a first embodiment of the invention the said water essentially freeof electrolyte is added to the aqueous solution of sodium silicateduring a part of the period in the acidulation thereof after the valueof x in the composition of the sodium silicate designated by the formulainfluence in the first stage of acidulation the ultimate or primaryparticle size of silica pigment produced as well as to influence thesize of network particles and the size of floc of the finallyprecipitated pigment relative thereto.

In a second embodiment of the invention, water essentially free ofelectrolyte is added after an x-value of at least about 4.2 is reached,and preferably only when the Tyndall effect has become evident. By thisembodiment it is possible to control the size of the primary particlesformed in the first stage essentially either by the control of theconcentration and temperature and rate of acidulation of the aqueoussodium silicate solution alone, or with a measured quantity ofelectrolyte, and to then influence the size of the network particles andthe size of floc of the final precipitate in relation to the size of theprimary particles.

In the third embodiment of the invention, water essentially free ofelectrolyte is added after the precipitation of silica pigment hascommenced. By this third embodiment it is possible to control the sizeof the primary particles and the size of the network particles asaforesaid, and to then influence the size of floc of the precipitaterelative to both.

In a fourth embodiment of the invention, the procedures of selected onesof the first, second and third embodiments are combined, so as toestablish the network particle size and the size of floc of the pigmentrelative to the size of the primary particles thereof in accordance withthe combined procedures.

In short, I have found that by applications of the present invention onecan vary the size of the ultimate or primary particles when desired, andcan vary relative thereto the size of the network aggregates thereofconstituting the pigment particles, and that by other applications ofthe invention one can vary the size of floc of the pigment relative tothe size of the network aggregates or pigment particles themselves, thusto prepare flocs well adapted for dispersion of the silica pigment tothe extent required in the situs of use thereof, and which have beenfound, when incorporated in elastomers, to produce vulcanizates withimproved properties, e.g. reduced heat buildup and/ or increased tensileproperties.

The present invention may be applied in the acidulation of alkali metalsilicate solutions having concentrations of alkali metal silicatedesignated by the formula M (SiO in which M is alkali metal-in the rangeof about 20 grams per liter to about 200 grams per liter, and beingsubjected to acidulation at temperatures between the freezing point andboiling point of water, i.e. from about 5 C. to about 100 C. atatmospheric pressure, or up to 200 C. or more if conducted under higherpressures.

After the fiocs of precipitated silica pigment have been formed they maybe recovered from the mother liquor in any suitable or preferred manner,e.g. by centrifuging, filtering, etc. and the silica pigment materialmay be removed as wet cake for further use with or without any furtherafter-treatment.

The carbon dioxide gas employed in this invention may be full strengthor may be diluted with air or other inert gases, e.g. such as the dilutecarbon dioxide gas produced by the combustion of hydrocarbons such aspropane or butane.

The rate of introduction of the carbon dioxide gas establishes the rateof acidulation of the sodium silicate and this rate may vary from themaximum rate of absorption of carbon dioxide by the sodium silicatesolution to rates one-tenth or even as slow as one-hundredth of thismaximum absorption rate. Application Serial No. 422,144, now Patent No.3,250,594 discloses the advantages of varying the rate of acidulation ofsodium silicate with carbon dioxide during certain stages of theacidulation and these procedures can be employed in conjunction with theteachings of this invention.

The process of this invention may be conducted in a batchwise orstepwise manner, or continuously, depending on available equipment.Suitable apparatus, for example, is set forth in copending applicationSer. No. 142,668, in which, for example, the present invention can bepracticed as a continuous process wherein the acidulation is applied tomoieties of the alkali-metal silicate solution in a series of zones,preferably coordinated with the periods of addition or non-addition ofother materials, with the addition of the added water effected in one ofsaid zones, or in a number of said zones less than all of said zones, inaccordance with the correlations of water additions and acidification ofthe solution contemplated by the present invention.

In the examples herein, like after-treatments are employed in each ofthe examples of the respective series set out to exemplify the inventionand its advantages, but the particular after-treatments are not claimedherein, and the novel ones thereof are claimed in other applications.

The silica product as wet filter or centrifuge cake may bemaster-batched with latices of natural or synthetic elastomers and/orplastomers.

The novel features of the invention are set forth in the claims appendedhereto, but the practice of the invention itself will be more preciselyunderstood by reference to the following specific examples embodying thesame, which are to be considered as illustrative and not restrictive ofthe invention.

EXAMPLES Example 1 In this example 14,065 grams (20 moles Na O/(SiO of41 B. commercial sodium silicate was dissolved in 63 liters of water andplaced in a stainless steel reactor agitated by a propeller typestirrer. The reactor and sodium silicate solution were heated to C. andso maintained.

Through a tube reaching to the bottom of the reactor carbon dioxide wasallowed to flow into the agitated sodium silicate solution at relativelyconstant rates as set forth in Table I and after minutes 6.7 moles ofcarbon dioxide has been introduced and the partially acidified sodiumsilicate solution took on the blue Tyndall effect color. At this 120minute time 15 liters of water were quickly added (i.e. in less than 1minute) to dilute the silicate solution and the acidulation with carbondioxide was continued. Table I herein sets forth the various rates ofacidulation with carbon dioxide employed throughout the acidification.

TABLE I.ACID ULATION RATE Time, minutes C 0 moles Acidification,

(Cumulative) (Cumulative) percent (Cumulative) 1 Tyndall efiectnoticeable and added 15 liters of 80 C. water The silica slurry from thereactor was filtered and washed until the soluble salts were less than1% by weight of the dry solids content.

A 2500 grams sample of the washed silica slurry having a pH of 8.6 wasdiluted with 1 liter of water and while agitating was treated with ml,of 10% aluminum sulfate with reduced to pH to 4.0. The so treated silicaproduct was filtered and the filter cake weighed 1370 grams afterwashing with 2 liters of water. The wet silica filter cake was dried at105 C. and the yield was 235 grams which after micropulverizing wasdesignated silica 1A.

A further 2500 grams of the washed silica slurry was treated with 70 ml.of 10% sulfuric acid and the pH was 4.0. This acidic silica slurry wasfiltered and washed with 2 liters of water and yielded 1040 grams offilter cake which on drying at 105 C. and micropulverizing yl iellgded215 grams of dry silica pigment designated silica Example 2(c0mparativeexample) had been consumed which was equivalent to an acidification of119%.

As in Example 1, the silica slurry so prepared was filtered and washeduntil the soluble salt content was below 1% and a 3000 g. portionthereof was treated with 138 ml. of aluminum sulfate and the resultingsilica. slurry had a pH of 4.0 and the silica product was filtered,washed with 2 liters of water, and the 1700 grams of 6 stock was agedovernight and then remilled and cured for 120 minutes at 287 F.

The vulcanizates were then tested and the physical properties thereofwere determined as set forth in Table III (the heat build-up beingdetermined with the Goodrich Flexometer in this table and in Table VI):

TABLE III Example Dilution Hardness Modules Tensile Elong. Heat Build-UpNo. (water, hter) (Shore A) (300%, p.s.i.) (Ult., p.s.i.) (percent) (AH,C.)

filter cake dried at 105 C. to yield 255 grams of product which aftermicropulverizing was designated silica 2A.

A further 3000 g. portion of the said Washed silica was treated with 62ml. of 10% sulfuric acid until a pH of 4.0 was obtained and the silicaproduct was filtered and washed with 2 liters of water and the resulting1790 grams of filter cake on drying yielded 264 grams of dry productwhich after micropulverizing was designated silica 2-B.

Example 3-(cdmparative example) This silica was prepared like Example 2except that the sodium silicate solution was made up in 78 liters ofwater instead of 63 liters of water.

The reactor charge consisted of 14,065 grams of commercial 41 B. sodiumsilicate moles N21 O/(SiO and 78 liters of water. After the ingredientswere mixed and the temperature raised to 80 C. the carbon dioxide feedwas started. The carbon dioxide was fed at substantially constant rateand after 750 minutes the reaction was terminated, 24.8 moles of carbondioxide having been fed which was equivalent to an acidulation of 124%.

The silica slurry so prepared was filtered and repeatedly washed untilthe soluble salt content was less than 1% dry silica basis.

As in Example 1, a portion of the washed silica slurry was treated with10% aluminum sulfate to a pH of 4.5 filtered and washed, dried at 105 C.micropulverized and designated silica 3-A.

Another portion of this silica slurry was treated in exactly the samemanner except that 10% sulfuric acid was employed in place of thealuminum sulfate to lower the pH to 4.0. The resulting dry silicaproduct was designated 3B The silicas prepared in Examples 1-3 were eachcompounded with SBR-1502 in accordance with the compounding recipe givenin Table II.

TABLE II Compound ingredients: Quantities (wt. parts) In compounding thestock the selected silica pigment material was milled into the SEE-1502together with the antioxidant and other compounding ingredients and theFrom these physical test results it is apparent that the silica of thepresent invention as exemplified in Example 1, whether treated withaluminun sulfate (Example 1-A) or with sulfuric acid (Example 1-B)yielded in the vulcanizate a better balance of physical properties, andespecially a lower heat build up, than the silicas of the comparativeexamples similarly treated and compounded.

Example 4 This example was made in a manner exactly like Example 1except that 1 liter of water containing 292 g. (5 moles) of sodiumchloride was added initially to the 14,065 grams (20 moles Na O/(SiO ofcommercial 41 B. sodium silicate dissolved in 62 liters of water. Afterthe sodium chloride electrolyte and the dilute sodium silicate solutionswere thoroughly mixed, the re actor was heated to C. and whilemaintaining the reactor contents at 80 C. carbon dioxide was introduced.The carbon dioxide was introduced at a relative constant rate and afterminutes 5.1 moles of carbon dioxide had been added and the solution tookon the blue color of the Tyndall effect. At this time 15 liters of waterwere added and the acidification with carbon dioxide continued. Theacidulation rates are set forth in Table IV hereafter.

TABLE IV.ACIDULATION OF SODIUM SILICATE WITH CARBON DIOXIDE Time,minutes 0 02, moles Acidulatiou,

(Cumulative) (Cumulative) percent (Cumulative) 1 Tyndall effectnoticable and added 15 liters of 80 C. water.

After termination of the acidulation the silica slurry was filtered andwashed until the soluble salts were less than 1% dry basis.

Two portions of the silica slurry were treated as in Example 1; oneportion being acidified to a pH of 4.5 with 10% aluminum sulfate and thesilica product filtered, washed, dried and micropulverized anddesignated as silica 4-A and the other portion being acidified to a pHof 4.0 with 10% sulfuric acid and the silica product, filtered, washed,dried and micropulverized designated as 4-B.

Example 5 In this example 292 g. (5 moles) of sodium chloride weredissolved in 1 liter of water and mixed with 14,065 grams (20 moles NaO/(SiO of commercial 41 B. sodium silicate dissolved in 77 liters ofwater. The dilute sodium silicate solution containing the sodiumchloride was heated to 80 C. and acidulated with CO while agitating. Theacidulation of the sodium silicate was carried on until the blue colorof the Tyndall effect was noticeable and then 10 liters of water wereadded and the acidulation continued until the silica precipitate hadsubstantially formed and a further 10 liters of water added and theacidulation continued. The acidulation rates are set forth in Table Vhereafter.

1 Tyndall effect noticeable and added 10 liters of 80 0. water.

2 Substantial precipitate formed and added 10 liters of 80 C. Water.

A sample of the silica of this illustrative example which had beenwashed as in Example 1, and then acidified to a pH of 4.0 with 10%sulfuric acid and filtered, washed, dried and micropulverized, wasdesignated silica B.

Example 6 (comparative example) This comparative example was preparedlike Example 4 except that the addition of water during theacidification in accordance with this invention was omitted. Thus anamount of 292 g. (5 moles) of sodium chloride in 1 liter of water wasadded to 14,065 grams (20 moles Na O/(SiOQ of commercial 41 B. sodiumsilicate dissolved in 62 liters of water. After the sodium chlorideelectrolyte and the dilute sodium silicate solutions were thoroughlymixed, the reactor was heated to 80 C. and while maintaining the reactorcontents at 80 C. carbon dioxide was introduced. The carbon dioxide wasintroduced at a relative constant rate and after 615 minutes 25.1 molesof carbon dioxide had been absorbed and the reaction was terminated.

Upon termination of the acidulation the silica slurry was filtered andwashed until the soluble salts were less than 1% dry basis.

A portion of the washed silica slurry was treated with aluminum sulfateuntil a pH of 4.5 was reached and the product was filtered, washed,dried, micropulverized and designated 6A.

Another portion of the washed silica slurry was treated with 10%sulfuric acid until a pH of 4.0 was reached and the product wasfiltered, washed, dried and micropulverized and designated 6B.

Example 7 (comparative example) This comparative example was preparedlike Example 5 except that the additions of water during theacidification in accordance with the present invention were omitted.

Thus, a solution was prepared of 292 grams of sodium chloride and 14,065grams moles Na O/(SiO of commercial 41 B. sodium silicate dissolved in78 liters of water and the solution was heated to C. In a period of 205minutes 24.8 moles of carbon dioxide were consumed during theacidulation.

The washing and after-treatment were as in Example 5, and the resultingsilica pigment was designated 7A.

The silicas of Examples 4A, 4-B, 5-A, 6A, 6B, and 7-A, were compounded,aged, cured, and tested in the same manner as set forth in connectionwith those of Table 111 above, and showed the physical properties setforth in Table VI.

From a comparison of these physical test results it is apparent that thesilicas of the present invention as prepared in Examples 4 and 5imparted to vulcanizates better tensile strength values withsubstantially no increase in heat build-up, as compared with the silicasprepared in the comparative Examples 6 and 7 similarly treated andcompounded.

From the examples given it will be appreciated that in the broaderaspects of the invention the water may be added (a) all at one time onthe appearance of a Tyndall effect, or (b) all at one time afterprecipitation has commenced, or (c) in a plurality of portions e.g.partly when the Tyndall effect occurs and partly when substantialprecipitation has occurred, or (d) at selected periods after theacidulation and building of the ultimate particles has progressedsubstantially, i.e. after x has reached a value of 3.75. The addition ofthe water all at one time, or in relatively few portions eachcorrellated with the x-v-alue of the solution is preferred forsimplicity, but the addition may be made more or less rapidly, overcorrespondingly more or less extended periods of time, so long as suchadditions are correllated with the stages of acidulation in a manner toinfluence the network particle size and/ or the size of fioc of thepigment relative to the primary particle size thereof. It is also to beunderstood that the present invention can be employed concurrently withthe variation of other conditions of the acidulation to obtain theadvantages of the present invention combined with those of other productcharacteristic controlling procedures.

The silica pigments contemplated herein are at least for the most partcomprised of silica. They usually comprise a few percent of free waterremovable by heating at C. and a few percent of bound water removable byheating at over 1000 C. They may contain a small amount of bound alkali.They may also contain a small amount of metal in the form of oxide,hydroxide, silicate or other salts, especially metal of the groupconsisting of magnesium, calcium, barium, zinc and aluminum. These andother metallo-ingredients of the silica pigments may be introduced, forexample, as described in Burke et al., US. Patent No. 3,178,388 datedApril 13, 1965 and in copending Burke et a1. application Ser. No.422,455, filed December 30, 1964. Thus the term silica pigmentcontemplates not only the pigments comprised solely of SiO but also thesilicious pigments containing proportions of other constituents as justdescribed.

While there have been described herein what are at present consideredpreferred embodiments of the invention, it will be obvious to thoseskilled in the art that minor modifications and changes may be madewithout departing from the essence of the invention. It is therefore tobe understood that the exemplary embodiments are illustrative and notrestrictive of the invention, the scope of which is defined in theappended claims, and that all modifications that come within the meaningand range of equivalents of the claims are intended to be includedtherein.

I claim:

1. In the preparation of silica pigment by the acidulation of an aqueoussolution of alkali metal silicate with the aid of carbon dioxide, theimprovement which comprises adding water essentially free of electrolyteto the solution during that part of the period in the acidulationthereof in which the value of x in the composition of TABLE VI ExampleDilution Hardness Modules Tensile Elong. Heat Build-Up No. (water,liter) (Shore A) (300%, 13.5.1.) (Ult., p.s.1.) (percent) (AH, C.)

4- 63/78 58 710 3, 540 740 44 4-B 63/78 58 820 3, 300 580 44 5-A78/88/98 56 700 3, 710 690 39 6-A 63 55 775 2, 065 610 43 6- 63 55 9702, 210 595 39 7- 78 66 825 3, 005 600 47 the alkali metal silicatedesignated by the formula M O(SiO wherein M is alkali metal-has becomegreater than 3.75; said Water being added in the proportion of fromparts to 150 parts per 100 parts of water present just prior to suchaddition.

2. In the preparation of silica pigment by the acidulation of an aqueoussolution of alkali metal silicate with the aid of carbon dioxide, theimprovement which comprises adding water essentially free of electrolyteto the solution during that part of the period in which the acidulationthereof in which the value of x in the composition of the alkali metalsilicate designated by the formula M O(SiO Wh6I6lI1 M is alkalimetal--has become greater than 4.2; said water being added in theproportions of from 10 parts to 150 parts per 100 parts of water presentprior to such addition.

3. In the preparation of silica pigment by the acidulation of an aqueoussolution of alkali metal silicate with the aid of carbon dioxide, theimprovement which comprises adding water essentially free of electrolyteto the solution during that part of the period in the acidulationthereof in which the precipitation of the silica has becomesubstantially complete; said water being added in the proportion of from10 parts to 150 parts per 100 parts of water present prior to suchaddition.

4. The invention of claim 1, wherein the addition of water essentiallyfree of electrolyte is terminated before the value of x reaches 5,3.

5. The invention of claim 1, wherein the addition of water essentiallyfree of electrolyte is terminated on the appearance of a Tyndall effect.

6. The invention of claim 1, wherein the addition of Water essentiallyfree of electrolyte is terminated when the precipitation of the silicahas become substantially complete.

7. The invention of claim 2, wherein the addition of Water essentiallyfree of electrolyte is initiated on the appearance of a Tyndall effect.

8. The invention of claim 7, wherein the addition of Water essentiallyfree of electrolyte is terminated when the precipitation of the silicahas become substantially complete.

9. The invention of claim 2, wherein a portion of said water essentiallyfree of electrolyte is added upon the appearance of a Tyndall effect andanother portion of said water essentially free of electrolyte is addedafter the precipitation of the silica commences.

10. The invention of claim 1, practiced as a continuous process whereinthe acidulation is applied to moieties of the alkali metal silicatesolution in a series of zones, and the addition of water essentiallyfree of electrolyte is effected in less than all of said zones.

11. The invention of claim 2, practiced as a continuous process whereinthe acidulation is applied to moieties of the alkali metal silicatesolution in a series of zones, and the addition of water essentiallyfree of electrolyte is effected in less than all of said zones.

12. The invention of claim 3, practiced as a continuous process whereinthe acidulation is applied to moieties of the alkali metal silicatesolution in a series of zones, and the addition of water essentiallyfree of electrolyte is effected in less than all of said zones.

References Cited by the Examiner UNITED STATES PATENTS 2,601,235 6/1952Alexander et al, 23-182 2,940,830 6/1960 Thornhill 23-182 3,250,5945/1966 Burke et a1 23182 OSCAR R. VERTIZ, Primary Examiner.

A. GRIEF, Assistant Examiner.

1. IN THE PREPARATION OF SILICA PIGMENT BY THE ACIDULATION OF AN AQUEOUSSOLUTION OF ALKALI METAL SILICATE WITH THE AID OF CARBON DIOXIDE, THEIMPROVEMENT WHICH COMPRISES ADDING WATER ESSENTIALLY FREE OF ELECTROLYTETO THE SOLUTION DURING THAT PART OF THE PERIOD IN THE ACIDULATIONTHEREOF IN WHICH THE VALUE OF X IN THE COMPOSITION OF THE ALKALI METALSILICATE DESIGNATED BY THE FORMULA M2O(SIO2)X-WHEREIN M IS ALKALIMETAL-HAS BECOME GREATER THAN 3.75; SAID WATER BEING ADDED IN THEPROPORTION OF FROM 10 PARTS TO 150 PARTS PER 100 PARTS OF WATER PRESENTJUST PRIOR TO SUCH ADDITION.